1. Field of the Invention
The invention relates to a multi-band antenna, and particularly to a multi-band antenna capable of operating at various wireless communication frequency bands and eliminating parasitic effect.
2. The Related Art
A portable communication device has an antenna that supports wireless communication in multiple operating frequency bands, such as global system for mobile communications (GSM). Wireless communication bands include global system for mobile communications (GSM) band about 850 mega-hertz (MHz), extended global system for mobile communications (EGSM) band about 900 MHz, digital cellular system (DCS) band about 1800 MHz and personal conferencing specification (PCS) band about 1900 MHz.
Many different types of antennas for the portable communication device are used, including helix, monopole, inverted-F, dipole, patch, loop and retractable antennas. Helix antenna and retractable antenna are typically installed outside the portable communication device. Inverted-F antenna, monopole antenna, patch antenna, loop antenna and dipole antenna are typically embedded inside the portable communication device case or housing.
Generally, embedded antennas are preferred over external antennas for the portable communication device owing to mechanical and ergonomic reasons. Embedded antennas are protected by the portable communication device case or housing and therefore tend to be more durable than external antennas. The volume of dipole antenna is large than the volume of monopole antenna because dipole antenna has a positive radiating body and a negative radiating body but monopole has one radiating body.
The volume of patch antenna is smaller than the volume of monopole antenna because monopole antenna is bent to form patch antenna. However, capacitance effect of patch antenna is increased because patch antenna is bent. For the purpose of eliminating capacitance effect of patch antenna, inverted-F antenna has a short portion connected to ground. Loop antenna is similar to dipole antenna having a positive radiating body and a negative radiating body. However two ends of loop antenna connect signal transmitter and ground to form an electric path. Therefore, gain effect of loop antenna is better than gain effect of dipole antenna. The volume of inverted-F antenna is smaller than other type antennas.
In recently, the portable communication device such as a mobile phone for downsizing issue contains problems both at closed state and opened state as well. The mobile phone is operated at wireless communication in opened state, else in closed state. When the mobile phone is in opened state, the movable housing of the mobile phone relatively moves to the main housing of the mobile phone.
According to the movement of the main housing and the movable housing, a main printed circuit board received in the main housing and a sub printed circuit board received in the movable housing relatively move to each other to form parasitic effect to influence gain of antenna of the mobile phone.
Therefore, the antenna capable of operating at various wireless communication bands, eliminating parasitic effect and being configured in the portable communication device is a development point.